FD1D_ADVECTION_FTCS is a C++ program which applies the finite difference method to solve the time-dependent advection equation ut = - c * ux in one spatial dimension, with a constant velocity, using the FTCS method, forward time difference, centered space difference, writing graphics files for processing by gnuplot.
We solve the constant-velocity advection equation in 1D,
du/dt = - c du/dxover the interval:
0.0 <= x <= 1.0with periodic boundary conditions, and with a given initial condition
u(0,x) = (10x-4)^2 (6-10x)^2 for 0.4 <= x <= 0.6 = 0 elsewhere.
We use a method known as FTCS:
The FTCS method is unstable for the advection problem. One purpose of this example is to demonstrate that fact.
For our simple case, the advection velocity is constant in time and space. Therefore, (given our periodic boundary conditions), the solution should simply move smoothly from left to right, returning on the left again. Instead, because of the instabilities, we see that the solution quickly becomes dominated by erroneous oscillations.
There are more sophisticated methods for the advection problem, which do not exhibit this behavior.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
FD1D_ADVECTION_FTCS is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version.
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GNUPLOT, C++ programs which illustrate how a program can write data and command files so that gnuplot can create plots of the program results.
Graphical output for this program was created using GNUPLOT. Data at selected time steps was written to a "data" file, and the appropriate GNUPLOT commands were written to a "command" file. The plot can be created by the command
gnuplot < advection_commands.txt
You can go up one level to the C++ source codes.